One of the more serious problems confronting a manufacturer of polyesterurethanes and manufacturers of parts prepared from polyesterurethanes is that degradation takes place over a period of time when conditions are such that hydrolysis can result. This degradation generally occurs at the ester linkage. Various techniques have been utilized in order to improve the stability of polyesters against hydrolysis. One such technique is to incorporate into the polyester either chemically or mechanically, compounds such as carbodiimide, alkylene carbonates, silicones and 2-imino-oxazolidines (see, e.g., U.S. Pat. Nos. 3,401,144 and 3,770,693).
Additionally, since many thermoplastic polyurethanes based on polyesters will ultimately discolor, only mild interest has been displayed in the past in providing products with low initial color. Whenever possible, care was taken to minimize color of the reactants, but to date, the thermoplastic polyesterurethanes themselves have not been assigned any color specifications. Recently, there has developed a market for colorless, transparent roller skate wheels molded from thermoplastic polyurethanes. The specific type of discoloration involved is that which occurred during exposure of the thermoplastic polyesterurethane to processing temperatures experienced during drying, extrusion or molding. The yellowing of the polymer did not seem to be associated with any significant change in strength properties and was entirely different from the discoloration which occurs during the service life of the ultimate product (i.e., weathering). It was found that this discoloration could occur during pelletization extrusion, resulting in the shipment of pellets which varied in color. It was also found that some lots of polymer discolored even more during the final product processing with the result that the final product also varied in color.